Cheers, Neptune, on your first 'anniversary'

Neptune's blue tinge, seen by Voyager 2, comes from a sprinkling of methane in its atmosphere, which is made mostly of hydrogen and helium (Image: Voyager 2 Team/NASA)

Dog years are nothing next to Neptune years. The distant planet has finally completed the first entire orbit of the sun since its discovery in 1846.

The planet is actually about as old as the solar system, which is 4.6 billion Earth-years old. But it makes sense to celebrate the "anniversary" of its discovery, since it was a veritable feat of astronomical deduction.

Astronomers predicted Neptune's existence and location based on the observed motion of Uranus, which was discovered in 1781. Astronomer Johann Gottfried Galle spotted Neptune from the Royal Observatory in Berlin right at the predicted location on 23 or 24 September 1846 (it was close to midnight and there is some uncertainty over the exact time). Galle is credited with its discovery, although Galileo appears to have seen it way back in 1612 and mistaken it for a star.

One full Neptune-year, which lasts 164.79 Earth-years, has now passed since its discovery. Figuring out the exact timing of this milestone is surprisingly complicated – it depends on the uncertain time of the discovery, and on exactly how you define "one Neptune orbit", since there are slightly different ways to do that.

But it falls around 12 July, give or take a day or two. To mark the occasion, we are singing the praises of one of the solar system's lesser-known worlds. Neptune, this is your life!

Neptune is the outermost of the eight official planets, in an orbit about 30 times larger than Earth's.

It has 17 times the mass of Earth and is thought to contain a rocky core surrounded by a thick blanket of hydrogen, helium, and water. Deep within the planet may lurk a weird form of water that behaves like both a solid and a liquid at the same time.

Neptune's gravity appears to have collected a huge quantity of asteroids or comet-like objects into its orbit.

NASA's Voyager 2 flew by Neptune in 1989. It returned beautiful images of the planet and its moons, including the largest and perhaps most intriguing moon Triton, which may be a captured cousin of Pluto.

More recently, Hubble Space Telescope images taken on 25 and 26 June show increasing cloud activity in Neptune's northern hemisphere, which may be tied to seasonal changes. Seasons last about 40 years on Neptune and it is now early winter in the northern hemisphere.

Another recent study reanalysed older Hubble imagery to clock Neptune's rotation period (that is, the length of its day) at 15 hours and 58 minutes, about 8 minutes shorter than previously thought.

While Neptune may seem exotic, its kin are as common as dirt in the wider galaxy. Neptune-size planets appear to be by far the most abundant type of planet in the galaxy, based on results from NASA's Kepler space telescope.